It's going to vary a LOT from gear to gear, and it depends on how much distortion you're willing to accept. In order to claim wider freq response, some manufacturers will cheat and allow incredible levels of distortion. They will also allow a large degree of non-linear response.

So while yes, it has "great frequency response", it sounds like CRAP.

I could tell you a typical response might be something like 60Hz - 15,000Hz, but again it's kind of meaningless as it depends on too many other parameters and assumptions in how the spec is obtained.

Here's a bit more info on it. I particularly like the bit where they say, "These limits are always set at the point where there is a 3 dB drop (on the lower and upper frequency limit)."

and immediately follow it with

"Unfortunately, this is not standardized,

and then

"...some manufacturers “cheat” a bit."

Actually....some "cheat" a LOT!

http://europe.beyerdynamic.com/service/faqs/kopfhoerer.html
What ist the Frequency Response?
The frequency response specifies the lowest and highest frequency that the acoustic transducer can reproduce. These limits are always set at the point where there is a 3 dB drop (on the lower and upper frequency limit). Unfortunately, this is not standardized, so it is often the case that some manufacturers “cheat” a bit. Whether headphones really have a frequency response of 20 to 20,000 Hz, for example, remains an open question. The question of what the curve of the frequency response looks like also remains open. Is it linear, or does it go up and down? You would be able to see this on the frequency response curve, but it is rarely included, since it looks pretty “horrible” for most headphones. This matter will be explained in another article. Ideally, you should really rely on your own hearing and simply try out headphones you are considering in a side-by-side comparison. The best way to do this is to compare the headphones to headphones with which you are already familiar.